The goal of this proposal is to develop innovative strategies based on novel chimeric Ig based proteins so as to prevent basophil and mast cell responses to FceRI crosslinking, understand the mechanisms involved and ultimately treat and interrupt the cycle of immunological events responsible for disease progression in atopy and asthma. Allergen specific and allergen non-specific chimeric proteins will be produced and evaluated. When IgE bound to the high affinity IgE receptor, FceRI, on tissue mast cells and peripheral blood basophils are crosslinked with specific antigen, degranulation causes the immediate phase of the allergic response. In addition, cytokines generated by activated mast cells and basophils help to maintain the Th2 environment that favors the late phase of this chronic disease. Activation signals mediated by ITAMs (immunoreceptor tyrosine-based activation motifs) in the FcepsilonRI are subject to negative regulation by the ITIM (immunoreceptor tyrosine-based inhibitory motifs)-containing receptor, including FcgammaRII. We have designed and tested one novel human gamma-epsilon Ig fusion protein which inhibits in vivo allergic responses and in vitro human mast cell and basophil functional and biochemical responses by FcgammaRII/FcepsilonRI co-aggregation. Our specific aims will test the hypothesis that molecules which co-aggregate ITIM receptors with FcepsilonRI will alter FcepsilonRI-signaling pathways and attenuate FcepsilonRI -mediated activation in vitro and in vivo. Aim 1 is to design and produce novel chimeric proteins that function as negative regulators of basophils and mast cells. This will include antigen non-specific molecules where direct binding to FcepsilonRI is provided by the epsilon heavy chain and ITIM binding employs a single or multiple gamma1 or gamma3 motifs. Antigen specific chimeric molecules comprised of allergen (e.g. Fel dl) plus an ITIM binding motif(s) will also be developed as a unique form of enhanced and safer immunotherapy. Aim 2 will test the ability of novel antigen nonspecific and specific molecules for their ability to inhibit human mast cells and basophils functional responses in vitro and to inhibit in vivo IgE mediated reactivity in transgenic mice and non-human primates. Aim 3 will dissect the biochemical mechanism by which such novel chimeric proteins negatively regulate FcepsilonRI- mediated functional responses using human mast cells and basophils. The results of these experiments will reveal whether ITIM-receptors and their downstream effectors are potential targets for innovative therapeutic interventions to manage allergic inflammation. Further, understanding of the mechanisms underlying the control of activating receptors by inhibitory receptors will have broad reaching implications for the manipulation the immune response.